1. Field of the Invention
This invention relates to concentrator systems and, more particularly, to digital concentrator systems for use in telephone subscriber loop digital carrier transmission systems.
2. Description of the Prior Art
It has been known for a long time that not all telecommunications users simultaneously demand service. Use has been made of this knowledge by terminating lines of low volume users of telecommunications services at a remote concentrator location, concentrating the traffic on fewer lines, and connecting the remotely located concentrator with the fewer number of lines to a central office. The first of these concentrators were electromechanical devices.
In more recent times, digital concentrators have been used. One such digital concentrator is described in U.S. Pat. No. 4,001,514 granted to Mr. W. K. Wurst on Jan. 4, 1977. There, a method of concentrating a large number of lines onto a fewer number of channels is disclosed. Wurst accomplished concentration by storing addresses of the lines assigned to channels, converting analog samples from assigned lines to delta modulated signals, and multiplexing thirty-two of the modulated signals along with three signals for supervision, timing, and other control functions, on a broadband digital transmission facility.
Additionally, digital transmission systems, such as the twenty-four channel carrier system called the "T-1" system, have been used between a remote location and the central office. One such T-1 carrier system, using so called "D-type" channel units, is disclosed in U.S. Pat. No. 4,059,731 granted to Mr. J. H. Green et al on Nov. 22, 1977. While reducing the number of wires used, these digital systems reserve a separate time-slot for each line, i.e., operate without traffic concentration.
These "T" carrier digital systems are in widespread use today. The use of systems according to the above-mentioned Wurst patent would necessitate the abandonment of the existing "T" carrier systems, thereby resulting in a considerable economical disadvantage.
It would therefore be desirable to provide a method of concentration using existing digital transmission systems.